This invention relates to a bipolar electrodeposition process of making catalysts, by which such catalysts may be engineered to contain a desired catalytic material or material on a predetermined location or locations of the supporting particles using site-selective (toposelective) bipolar electrochemistry.
More specifically, the invention relates to an electric-field process to prepare electrodeposited catalysts which does not require physical contact between the power source and the deposits. As will be described hereinafter, such an approach, referred to as bipolar electrodeposition, allows not only for electrodeposition of catalytic material within several centimeters of a supporting structure for the catalytic material, but also has the additional advantage that the localization of the deposit can be controlled. Even more particularly, metal nanostructures are electrodeposited onto electrically isolated conductive particles using bipolar electrochemistry. The principle relies on the polarization of the conductive particles by applying an electric field, which results in the electrodeposition of a metal salt onto the cathodic region of the particles. The result is the creation of nanoscale metallic structures at the surface of colloidal particles with spatial control without using masks, templates or physical contact with the particles.
Although catalysis and methods of making catalysts are generally well known, new environmental concerns continue to challenge these fields, for example, in providing less expensive hydrogenation processes, and in more efficient control of CO and NOxemissions. There are many known methods for producing catalysts, including, for example, thermal reduction of metal salts on supports such as alumina or titania; adsorption of preformed colloidal metal dispersions onto a support; electrodeposition of metals onto conductive supports, such as carbon, metals or conductive oxides or onto high surface area organic polymers; and photoelectrodeposition onto semiconducting support particles.
These techniques have certain limitations, in that several are not particularly well-suited for making extremely small catalyst particles, such as nanostructure particles (sub- 100 nm scale). Moreover, since catalysis is affected by both the size of the catalyst particles and their morphology, better techniques are needed to control the morphology of such small catalyst particles.
Nanostructure materials frequently exhibit properties deviating significantly from the bulk. This has been especially true in the field of catalysis, where catalytic activity and selectivity have exhibited a profound dependence upon the size and morphology of the catalyst. Both geometric (physical registry of the adsorbate with the atomic positions on the catalytic surface) and electronic (e.g. change in local electron density or conductivity) effects have been proposed to explain such behavior.
Electrodeposition is potentially a very attractive process of preparing nanostructure materials. One successful approach is electrodeposition within nanoscale templates such as porous alumina, polycarbonate membranes, within electron-beam patterned areas, at interfaces, defects within monolayers, or in the areas adjacent to a scanning tunneling microscope tip. The advantage of such a strategy is that the electrodeposited material can be prepared with excellent shape and size control, dependent upon the structure of the template. A major drawback is the necessity to obtain templates of the desired geometry.
A way of avoiding the use of templates to control electrodeposit size and morphology is to manipulate the current and voltage parameters during electrodeposition. This type of approach has been used extensively to prepare electrocatalysts. The following conditions are most often used to control the size and morphology of nano-electrodeposits.
Potentiostatic or galvanostatic conditions. In general high voltages and high currents favor smaller particles because, above the nucleation overpotential, the growth of new centers is favored at the expense of the growth of existing nuclei. For example, changing the current density from 0.1 to 5 mA/cm2 can control Pd cluster size from 4.8 to 1.4 nm.
Low frequency ( less than 0.1 Hz)cycling usually using cyclic voltametry. Changing sweep rates from 1 to 10 mV/s controlled the size of silver deposits from 93 to 18 nm. It was postulated that faster sweep rates encouraged more rapid nucleation, thus leading to smaller particles.
Pulsed Electrodeposition. Here, square pulses are applied at a desired frequency. Particle size can be easily controlled by varying the ON or the OFF time of the pulse sequence. When the pulse is ON nucleation and growth occur. While the pulse is OFF, unstable small nuclei dissolve leaving the larger nuclei to continue to grow during the subsequent ON times. Thus increasing either the ON or OFF times during pulsed electrodeposition generally leads to increases in particle size. For example it was possible to prepare nanocrystalline palladium deposits with sizes ranging from 18 to 35 nm for ON-times of 1 to 4 ms and from 20 to 95 nm for OFF times increases from 40 to 200 ms. A related technique is reversing current deposition, where smaller particles are preferentially dissolved during the anodic pulse. However, other researchers have found opposite effects and it has been postulated that when the pulse is OFF, inhibiting species have the time to passivate the deposits for further growth, thus favoring smaller particles with longer OFF times. For example, copper deposits from 10 to 83 nm can be obtained by varying the OFF times from 100 ms to 6 ms with 1 ms ON times. Regardless of the direction of the effect, it is clear that particle morphology and size on the nanoscale can be controlled by pulsed electrodeposition with ON and OFF times on the 1-200 ms time scale.
Waveform modulation. It has also been observed that the waveform has a profound influence on the deposit morphology. For example, at 100 Hz, copper deposition size increase in the following order: square-wave greater than sinusoidal greater than triangular.
In principle, electrodeposition offers a rather convenient method of modulating the properties of metal catalysts. Indeed, electrodeposition has been used extensively to prepare metal catalysts on conductive substrates such as carbon, metals or conductive oxides. Underpotential deposition to form surfaces with modified catalytic behavior could be considered a related technology. One of the critical limitations of electrodeposited catalysts is that the deposited area is essentially limited to the electrode surface. In order to increase the available surface area of electrodeposited catalysts, electrodeposition of metal structures within polymers such as polyvinylpyridine, polyvinylacetic acid, and NAFION(copyright) has been actively explored. A recently developed related approach is electrodeposition within thin gel coatings. However, such approaches are limited to coatings of only a few micrometers thick because of the difficulty of assuring an ohmic or electron hopping contact with the growing electrodeposit while maintaining a highly permeable structure for rapid reagent diffusion and high surface area. Furthermore, small isolated deposits cannot usually be obtained by this method because of the necessity of having an electrically contiguous structure from the electrode extending into the matrix.
Other attempts to increase the thickness of electrodeposited catalysts include the use of conductive polymers such as polypyrrole, polyaniline or viologen-based polymers, since electron conduction can occur through the polymer. However, due to anisotropic field distributions and the finite resistivity of the conductive polymer, it is unlikely that homogeneous electrodeposition will be possible within volumes more than a few micrometers thick using this approach. Furthermore, the choice of support is limited to conductive polymeric materials in electrical contact with an electrode.
One electrodeposition tactic that avoids electrical contact with the conductive support is photoelectrodeposition onto semiconductive particles. This has been used extensively as a method of electrodepositing catalytically active metals (e.g. Au, Pt, Pd, Ag, Rh, Ir) onto a dispersed semiconducting support (e.g. TiO2, ZnO, SnO2, ZrO2, ThO2, CdS, WO3). In this case photons promote electrons from the valence band into the conduction band of the semiconducting particles, creating a situation where anodic and cathodic processes occur on different regions of the same particle. Although this method has been successful in producing catalysts, it requires the use of a semiconductor with a bandgap tuned to the wavelengths capable of penetrating into the sample. Furthermore, due to absorption of light, homogeneous exposures within large volumes is not possible without prolonged mixing.
In all of the above methods except photoelectrodeposition, an ohmic contact between the electrodeposit the electrode is necessary. A crucial concept in the present invention is the elimination of this requirement by the use of bipolar electrochemistry, which is described in more detail hereinafter. Another crucial concept in the present invention is to control the properties of the catalysts and catalytic systems containing the catalyst particles by modulating electric field parameters during their preparation. Thus, in accordance with the present invention, control of the electric field parameters involved in the bipolar electrochemical process of making the catalysts, particularly the direction and intensity of the applied field among others set forth hereinafter, is responsible for creating a catalyst having desired predetermined structure and properties.
One aspect of the present invention relates to a bipolar electrochemical process for toposelective electrodeposition of a catalytic substance on an electrically conductive particulate substrate comprising: (a) placing the conductive particulate substrate and a source of the catalytic substance into an environment capable of conducting electricity; (b) aligning the conductive particulate substrate on which the catalytic substance is to be deposited with respect to an electric field such that the conductive particulate substrate is not in physical contact with electrodes and such that the catalytic substance will be deposited in a predetermined location on the particulate substrate when an electric field is applied; and (c) creating an electric field of a sufficient strength and for a time sufficient to deposit the catalytic substance from the source of the catalytic substance on the conductive particulate substrate at the predetermined location in substantial alignment with the electric field.
Another aspect of the present invention relates to the various catalysts produced by the process of the present invention.